This invention relates generally to screens for displays including screens over direct view displays and screens upon which an image is projected in projection displays.
Displays are commonly used to show an image which has been electronically generated. Displays are found in televisions receivers, computer monitors and projection display systems, processor-based appliances, toys and games and in a variety of hand-held devices including personal digital assistants, telephones, and the like. These displays may generally be divided into direct view displays, in which an image is viewed directly by the user, and projection displays, in which the image may be projected onto a larger surface so that a larger image results which is more readily viewable by one or more viewers. Projection displays may include those that use reflective techniques and those that use transmissive techniques for generating the projected image. In each case, a screen ultimately provides the image for view by the user.
Display screens are subject to a number of shortcomings. Commonly, the display screen may produce glare as a result of the reflection of ambient light from flare and ghosting. In addition, displays may be subject to graininess wherein the particles forming the display screen or the imaging device itself create pixel boundaries which are readily viewable by the user. Some displays are subject to a defect called pixellation where the graininess of the imaging device is visible to the user.
In addition, some displays, including those that are using coherent light sources, may be more susceptible to interference effects resulting in speckle. Speckle is a plurality of point light images which come from the display and may move when the user moves with respect to the display. To some degree, speckle artifacts may be enhanced by the graininess of the imaging device or the display screen.
Some displays may have limited fields of view so that the viewing angle may be restricted. If the user moves beyond the viewing angle of the display, contrast reversal may result.
It is known to use a diffuser over display screens to improve the viewing angle and to overcome the graininess of the image that is viewed. For example, some display screens use a diffuser in combination with a Fresnel lens to overcome these effects. However, such systems do little to overcome the effects of ambient light on the display and the glare that may result. In addition, many of the techniques for overcoming graininess and speckle are less than completely effective.
More recently, rear projection television screens have been based on a laminate structure combining lenticular and Fresnel optical elements that may achieve increased brightness uniformity, increased image brightness in a preferred viewing cone and reduced glare. A Fresnel lens forms an image of the projection lens aperture in the middle of the viewing area which is typically a small region. This small region is enlarged by adding a weak diffuser which expands the viewing area along the vertical direction. A lenticular array stretches the viewing area along the horizontal dimension. A holographic diffuser can replace a conventional diffuser and a lenticular array.
While existing designs may provide adequate gain, brightness and contrast, they may not provide adequate ambient light rejection due to back reflection from the screen. In some cases, speckle may be a problem due to the finite grain size on the diffusing screen, which may be a particularly severe problem under coherent illumination.
Thus, there is a continuing need for techniques which provide adequate gain, brightness and contrast while improving the ambient light rejection and, in some applications, reducing speckle.